In recent years, as one of methods of achieving a miniaturized and high-performance semiconductor device, a mounting technique of laminating semiconductor substrates (hereinafter, referred to as silicon substrates) each made of monocrystalline silicon and electrically connecting the silicon substrates to each other by using a fine electrode wiring has been developed.
Among the above-described mounting techniques, a method on which has been recently focused is a method of electrically connecting integrated circuits to each other, which are formed on a plurality of silicon substrates, by using a fine electrode which is called bump electrode for connecting the silicon substrates to each other and using an electrode which is called Through Silicon Via penetrating through the silicon substrates.
In a case of a semiconductor device to which the above-described method is applied, connection reliability of the bump electrode for connecting the silicon substrates to each other is degraded due to the stress such as heat or mechanical impact applied to the silicon substrates. Therefore, a technique is also required, in which the bump electrode is protected by sealing a periphery of the bump electrode with an insulator such as a polymer for ensuring the connection reliability of the bump electrode.
A solder material containing tin (Sn) or lead (Pb) as a main component is currently widely used as a bump electrode material formed on a silicon substrate. The integrated circuits which are formed on different silicon substrates from each other can be electrically connected to each other by heating the bump electrode which is formed on the silicon substrate and which is made of the solder material up to a melting point of the solder material or higher and bonding the bump electrode to an opposite bump electrode which is formed on a substrate to be connected. Also, for the protection of the bump electrode by using such a solder material, generally, an Epoxy-based polymer is used due to limitation of a process temperature, and fine particles of silicon oxide called filler are added thereto in order to adjust a coefficient of thermal expansion.
However, the melting point of the solder material is low, and therefore, there is a problem of low electrical reliability for a long period. For example, a melting point of a tin (Sn)-silver (Ag)-copper (Cu) material, which is such a lead-free solder material as currently widely used for electronic components, is about 220° C. This temperature is much lower than a melting point (660° C.) of aluminum (Al) which is widely used as an interconnection of an integrated circuit, and therefore, there is a problem that electrical connection failure tends to occur due to a current called Electromigration. More particularly, under a state that the bump electrode is miniaturized to cause high current density, the reliability is seriously decreased.
Accordingly, as the material of the bump electrode, a method of using a metal material having high Electromigration reliability has been proposed.
For example, U.S. Patent Application Laid-Open Publication No. 2007/0207592 (Patent Document 1) discloses a method of using copper (Cu) as the material of the bump electrode. A melting point of copper is as high as 1084° C., and it is known that its Electromigration reliability is higher than that of aluminum. From this view point, even if the bump electrode is miniaturized to cause the high current density in the bump electrode, the decrease in the Electromigration reliability of the device with using the bump electrode mainly containing copper can be prevented, so that the connection is possible. Hereinafter, the method described in Patent Document 1 is referred to as conventional example 1.
Japanese Patent Application Laid-Open Publication No. 2003-100801 (Patent Document 2) describes that a bump electrode and a dummy bump are formed on one silicon substrate, a metal wiring and a protection film are formed on the other silicon substrate, the bump electrode is bonded to the metal wiring and the dummy bump is bonded to the protection film when the silicon substrates are stuck to each other, and, after the bonding, a polymer for the bump protection is injected. Hereinafter, the method described in Patent Document 2 is referred to as conventional example 2.